Electric cables

ABSTRACT

A conductive or semiconductive fibrous sheet material used as an electric screen in contact with the dielectric fluid of high voltage cables having a fluid impregnated dielectric is loaded both with conductive carbon and with at least one active, finely divided material selected from the group consisting of aluminium oxide and such other metallic oxides, hydrated metal oxides, metal hydroxides, metal carbonates and basic metal carbonates as have adsorptive powers comparable with that of active finely divided aluminum oxide. The dielectric is free of any adsorbent.

ELECTRIC CABLES This is a division of U.S. Pat. application Ser. No.20,670, filed Mar. 18, 1970 now abandoned.

This invention relates to conductive or semiconductive fibrous materialin sheet form suitable for use as an electric screen in contact with thedielectric fluid of high-voltage cables and other electrical apparatushaving a fluid impregnated dielectric.

The invention is especially concerned with the dielectric and conductorscreens of oil-filled electric cables which may be of the pipe type orof the selfcontained type having one or more internal oil ducts. It hasas its principal object the provision of a permeable material for thispurpose that is capable of protecting the dielectric material againstcontamination and that is less liable to contaminate the dielectricand/or its impregnant than are permeable materials hitherto used.

The material in accordance with the invention is a conductive orsemi-conductive fibrous material in the form of a flexible sheet loadedboth with conductive carbon and with active finely divided materialselected from the group consisting of aluminium oxide and such othermetal oxides, hydrated metal oxides, metal hydroxides, metal carbonatesand basic metal carbonates as have adsorptive powers comparable withthat of active finely divided aluminium oxide. Of these materialsaluminium oxide and titanium dioxide are preferred, in that order.

I prefer to use aluminium oxide or other active substances in the mannerdisclosed in U.K. Pat. No. 874,981 except that instead of incorporatingthese substances in a paper or other dielectric material suitable foruse as a dielectric as claimed in that patnet, they are incorporated infibrous materials suitable for use as a conducting or semi-conductingscreen.

The invention includes cables, especially cables having an impregnateddielectric such as oil-filled cables, in which the fibrous materials areused either as a conductor screen or as a dielectric screen or both.Normally, at least two layers of the loaded fibrous material will beused in each screen, the layers breaking joint.

The invention also includes multi-ply fibrous sheet materials in whichat least one but not all plies is loaded with the active material andconductive carbon.

The invention also includes multi-ply fibrous sheet materials in whichat least one but not all plies is loaded with conductive carbon but notwith active alumina or other active material and at least one of theremaining plies is loaded with such active material but not withconductive carbon. in forming a conductor screen from such sheetmaterial, such sheet material may be applied over an underlying layer ofmaterial loaded with conductive carbon, with the ply or plies that is orare loaded with active alumina or other active material on the outside(that is adjacent to the dielectric) but to avoid exposure of theconductive carbon loaded paper to the oil in the butt spaces between theturns of the multi-ply sheet material, it is preferable to apply amulti-ply sheet material over an underlying layer of material loadedwith both active alumina and conductive carbon. A similar but invertedconstruction may be used in forming a dielectric screen.

The quantities and type of conductive carbon used will be substantiallythe same as in conventional carbon loaded paper used for this purpose.

The particle size of the alumina or otheractive substance must be suchthat it does notinterfere with the paper making process; colloidalparticles are preferably used. Although less than 2 percent of theactive substance based on the weight of the paper can be used, since itseffectiveness is a function of the quantity used larger quantities, e.g.8-10 percent, are preferred. The active substance must be substantiallyfree from ionic impurities that would deleteriously affect theelectrical properties of the dielectric and before incorporation in thepaper it is preferably subjected to the same tests as is paper for useas the dielectric of electric cables and other electrical apparatus,i.e., tests for conductivity and acidity/alkinility of the aqueousextract. Also, as stated in the above mentioned British PatentSpecification, the substance must not be overheated during theactivation process to the extent that fusion of the particles occurs,since this would materially reduce the adsorptive power and by virtue ofthe increased crystalinity and abrasiveness interfer with the felting ofthe paper fibres. The carbon and the alumina, or other active substance,can each be incorporated in the fibrous material either duringmanufacture, i.e., by addition to the pulp in the case of paper eitherat the hydro pulper or the beater, or after manufacture by impregnationof the fibrous material with a suspension of the material or materials.

Although the invention is mainly concerned with sheet material based onpaper of the quality normally used as the dielectric in high voltagecables, other fibrous sheet materials, both natural and synthetic, thatare compatible with the dielectric materials can alternatively be used.

The sheet material in accordance with the invention is intendedprimarily to act as a barrier inhibiting the migration into thedielectric of ionic impurities carried by the fluid impregnant, whichimpurities may for example be derived from metals with which the fluidcomes into contact within the electrical apparatus or in ancillaryapparatus through which the fluid is circulated, for example the metalof cable conductors, enclosing pipes or cable sheaths, binding tapeapplied to the dielectric, external heat exchangers, and circulatingpumps. The alumina or other active substance may also inhibit migrationof carbon or other semi-conducting particles derived from carbon blackfrom the fibrous material into the adjacent dielectric or the impregnanttherefor. In the accompanying drawings FIGS. 1 & 2 show two forms offibrous sheet material in accordance with the invention; and FIG. 3shows in cross-section a cable having a conductor screen and adielectric screen of the material in accordance with the invention.

The presence of the alumina or other active substance will greatlyincrease the adsorptive power for contaminating ions of the paper orother fibrous material by virtue of the much higher adsorptive index ofthe alumina or other active substance as compared with the activatedcarbon. The presence of the alumina or other active substance willhowever not substantially affect the permeability of the loaded fibrousmaterial and hence will not obstruct the passage of moisture from thedielectric during the drying of the dielectric prior to impregnation.

The sheet materials in accordance with the invention have the advantageover carbon loaded papers hitherto used that when used instead of suchmaterials as the conductor and/or the dielectric screen of a highvoltage cable with an oil impregnated dielectric they reduce the rate atwhich the power factor of the cable increases over a long period ofservice.

They have the advantage over screens of solid metal foils (usuallyaluminium the alloy sold under the trademark Aluman, copper or bronze)and of metal foil laminated with fibrous material that they arepermeable and that in comparison with the latter type of material therisk of contamination of the dielectric by the adhesive used to bond themetal foil to the fibrous material is avoided.

What I claim as my invention is:

1. An electric cable, comprising a. at least one metallic conductor,

b. a fluid impregnated dielectric surrounding said conductor and boundedat each of its inner and outer circumferential surfaces by an electricscreen in contact with said fluid and formed from a fibrous material inthe form of a flexible sheet loaded with conductive carbon black and c.an external fluid-tight sheath, said cable being characterized in thatthe said fibrous material of said electric screens comprises distinctplies loaded respectively with said conductive carbon black and with anactive finely divided adsorbent material selected from the groupconsisting of aluminum oxide and such other active metallic oxides, hy-

'drsted metal oxides, metal hydroxides, metal car bonates and basicmetal carbonates as have adsorptive powers comparable with that ofactive finely divided aluminum oxide and that the said dielectric isfree of any adsorbent material.

2. An electric cable, comprising a. at least one metallic conductor,

b. a fluid impregnated dielectric surrounding said conductor and boundedat each of its inner and outer circumferential surfaces by an electricscreen in contact with said fluid and formed from a fibrous material inthe form of a flexible sheet loaded with conductive carbon black and c.an external fluid-tight sheath, the said cable being characterized inthat the said electric screens each comprise a first layer of amulti-ply fibrous sheet material comprising one ply contiguous with thedielectric and loaded with an active finely divided adsorbent materialselected from the group consisting of aluminum oxide and such otheractive metallic oxides, hydrated metal oxides, metal hydroxides, metalcarbonates and basic metal carbonates as have adsorptive powerscomparable with that of active finely divided aluminum oxide and anotherply loaded with conductive carbon black and a sec-- ond layer ofmaterial loaded with conductive carbon black and contiguous with saidother play and further characterized in that the said dielectric is freeof any adsorbent material.

3. A cable as claimed in claim 2 in which said second layer is alsoloaded with an active finely divided material selected from the groupconsisting of aluminium oxide and such other metallic oxides, metalhydroxides, metal carbonates and basic metal carbonates as haveadsorptive powers comparable with that of active finely dividedaluminium oxide.

2. An electric cable, comprising a. at least one metallic conductor, b.a fluid impregnated dielectric surrounding said conductor and bounded ateach of its inner and outer circumferential surfaces by an electricscreen in contact with said fluid and formed from a fibrous material inthe form of a flexible sheet loaded with conductive carbon black and c.an external fluid-tight sheath, the said cable being characterized inthat the said electric screens each comprise a first layer of amulti-ply fibrous sheet material comprising one ply contiguous with thedielectric and loaded with an active finely divided adsorbent materialselected from the group consisting of aluminum oxide and such otheractive metallic oxides, hydrated metal oxides, metal hydroxides, metalcarbonates and basic metal carbonates as have adsorptive powerscomparable with that of active finely divided aluminum oxide and anotherply loaded with conductive carbon black and a second layer of materialloaded with conductive carbon black and contiguous with said other playand further characterized in that the said dielectric is free of anyadsorbent material.
 3. A cable as claimed in claim 2 in which saidsecond layer is also loaded with an active finely divided materialselected from the group consisting of aluminium oxide and such othermetallic oxides, metal hydroxides, metal carbonates and basic metalcarbonates as have adsorptive powers comparable with that of activefinely divided aluminium oxide.